Cobalt base alloy



sepf. 24, 1968 S. T. WLODEK COBALT BASE ALLOY Filed Aug. 31, 1964 ruin/fy- United States Patent O 3,403,021 v COBALT BASE ALLOY Stanley T. Wlodek, Cincinnati, Ohio,` assignor to General Electric Company, a corporation of New York Filed Aug. 31, 1964, Ser. No. 393,328 4 Claims. (Cl. 75-171) This invention relates to cobalt base alloys, and more particularly to an improved Co-Cr-W alloy of increased long time resistance to embrittlement.

A number of cobalt base alloys have been developed yfor sophisticated high temperature applications where ductility and Istrength are important. Such applications include alloys for turbine blades and sheet metal parts in the hot sections of jet engines. One of the strongest cobalt base alloys, presently widely used for wrought applications,`is an alloy sometimes referred to as L605 Alloy or as HAYNES Alloy No. 25 and having a composition, by weight of 19-2l% Cr, 14-16% W, 9- 11% Ni, 1-2% Mn, 0.05-0.15% C, up to 3% Fe and up to 1% Si with the balance Co. While this alloy has properties adequate for a large number of applications, some of the more sophisticated high temperature applications, such as for use in power equipment for outer space, places more stringent demandson the mechanical and metallurgical stability of such alloys. It has been recognized that in the temperature range 1200-2000 F., the precipitation of a phase sometimes referred to as Laves phase is the main embrittling process in commercially available L-605 alloy. This precipitating Laves phase has the probable formula (Co, Ni)2 (Cr, W). It was further recognized that the precipitation of M23C,` carbides may also contribute to embrittlement.

It is a principal object of this invention to provide an improved Co-Cr-W alloy based on cobalt and having improved resistance to embrittlementat temperatures up to about 2000a F. under vconditions placing stringent demands on the mechanical and metallurgical stability of the alloy.

This and other objects and advantages will be more clearly recognized from the following detailed description, example-s and the drawing, all of which are meant to be exemplary of rather than any limitation on the scope of the present invention.

In the drawing: l

FIG. l lis a graphical illustration'of the range of time and temperature in which an alloy within the scope of the present invention will lose ductility on prolonged exposure; p l,

FIG. 2 'is a graphical comparison of the loss of bend ductility after aging at 1600 F. for alloys both within and outside the scope of the present. invention; and

FIG. 3 is a graphical comparison of the effect of aging at 1600 F. on room temperature tensile properties.

Briefly, the present invention consists of an improved cobalt rbase alloy of the L-605 alloy type consisting essentially, by weight, of the elements Cr, W and Ni in a total amount of l2-48%, elements selected from Si and Zr, the Si when included being in an amount no more than 0.5%, and the Zr when included being in an amount no more than 0.2%, 0.05-0.15% C, up to Fe, about 1-2% Mn, w-ith the balance cobalt. In a preferred form, Fe is included in the range of 2-5%.

In order to more fully -understand the present invention, the following table is presented and includes alloy forms p 3,403,021 Patented Sept. 24, 1968 studied which are typically within and without the Scope of the present invention.

TABLE-WEIGHT PERCENT, BALANCE Co Heat Cr W Nl Fe Mn Si C It was recognized in connection with the types of alloys represented in the above table that on exposure in the temperature range of 1200-2000 F., the loss of ductility is associated with a precipitation reaction-that can produce, in time, a completely brittle intergranular failure. The structure of the original or as-received material was single phase. Upon aging, certain of the alloys of the table formunstable precipitates of Laves phase as well as the undesirable M23C6 car-bide which is known to contribute slightly to alloy embrittlement but not as much as the Laves phase'. Although the formation of M23C6 carbides can be related to the inclusion of higher amounts of chromium in nickel or cobalt base alloys, by maintaining the carbon level below about 0.1 weight percent, the formation of the desirable yMGC carbide is promoted rather than the undesirable .and embrittling M23C6 type carbide.

Thus, the alloy of the present invention contemplates the inclusion of no more than about 0.15 weight percent carbon to inhibit the formation of undesirable and embrittling carbide phases. More important, however, was the unexpected recognition that control of amounts of the element silicon, tolerated in known alloys of this type as an impurity or added as a deoxidizing element, is the most significant contribution to inhibiting (l) precipitation of unstable Laves phase and (2) loss of ductility in the type of alloys shown in the table. In addition, it has ybeen recognized that the inclusion of iron in the range of 2-5 weight percent in the presence of less than 0.5 weight percent S-i further contributes to the stability of the alloy.

Because of recognition of the unexpected stabilization of alloys including 0.5% Si or less, and because of the identication of an extremely stable Laves phase in the alloy of the present invention, an attempt was made to theorize on the mechanisms involved. No Laves phase has been found in either of the binary Co-W or Co-Cr phase diagrams. A speculative ternary phase diagram for the Co-Cr-W system also discounts the possibility of a Laves phase. In addition, the presently available rules of structural bonding that have been developed for the Laves form support the absence of this phase in the Co-Cr-W system. n

The formation of the Laves AZB structure requires a favorable combination of atom size and valence. In the Co-Cr-W system, the size factor -is favorable. The average electron concentration for the A2B combination is 8, which is slightly in excess of the maximum in the allowed range of the Laves phase. Elements which will reduce the electron concentration will, therefore, stabilize Laves phase in the Co-Cr-W system. Silicon (4e/A) has been reported to decrease the electron concentration sufficiently 3 to stabilize the Laves phase CoSSi W2. Zirconium (4e/A) may also have a similar effect because CozZr is a stable Laves structure.

The contention that Si and Zr can stabilize the Laves phase structure in the alloy types found in the table, explains the appearance and the stability of Laves phase in the alloy structure within the scope of the present invention.

Heats A, B and C in the table represent alloys within the scope of the preferred form of the present invention, including silicon at less than 0.5 Weight percent and iron in the range of 2-5 weight percent. Heat A represents the specific preferred form of the present invention including iron in the range of 3-5 weight percent. It is to be noted that the heats in the table are arranged in the order of increasing sensitivity to embrittlement. It was found that as embrittlement proceeds, the fracture becomes completely intergranular and there was marked absence of plastic deformation prior to fracture.

FIG. l is presented to show the range of time and temperature in which the alloy of heat A will lose ductility on prolonged exposure. This loss in bend ductility, which within 1000 hours can occur over the temperature range of l200-2000 F., is associated with the precipitation of unstable Laves phase. The present invention is directed principally to the stabilization of such phase. The bend ductility testing was performed on specimens 0.5" by 2" evaluated in 3 point loading (0.05 in./in./min.) over a 105 mandrel and a 1.5 T radius. In FIG. l, it is to be noted that the maximum rate of reaction is at l600 F. Most of the testing, therefore, was conducted by aging at this temperature.

The loss of bend ductility after aging at 1600 F. is presented in FIG. 2 for a number of heats. It is to be noted there that heat B, within the scope of the present invention, is significantly improved in this respect over heats D and G, both of which include Si in amounts greater than 0.5 weight percent and therefore are outside the scope of the present invention.

FIG. 3 is a comparison between heat A, within the scope of the present invention, and heat E, outside the scope of the present invention, with regard to room temperature tensile properties including tensile ductility of specimens aged at 1600 F. in air. It is to be noted that heat A maintains a higher room temperature tensile yield strength and ductility even after aging up to 1000 hours. The tensile properties represented in FIG. 3 were sheet tensile specimens of one inch gage length first aged and then pulled in tension at 0.005 in./in./min. in a 60,000 pound machine.

Although the data has shown that larger amounts of Fe in the range of 2-5 weight percent, as represented by heat A in the table, lends increased ductility to the alloy probably based on theories of stabilization of the facecentered-cubic (f.c.c.) form of the cobalt solid-solution matrix and the effect Fe and C have on the f.c.c.toh.c.p. (hexagonal-close-packed) transformation, nevertheless the main embrittling reaction in these types of alloys is the precipitation of Laves phase and the resultant increase in the yield strength. The grain boundary areas become weaker by comparison, thus providing the preferred zone of failure. The yield strength curves of FIG. 3 which indicate a constant yield strength and a small drop in ductility with aging time for heat A and an increase in yield strength that paralleled the most pronounced drop in ductility observed for heat E support this mechanism.

The deleterious effect of larger amounts (eg. above 0.5%) of Si can be associated with lack of stabilization of the Laves phase and of the h.c.p. cobalt structure as well as with the contribution of such structure to embrittlement. The possible beneficial effect of iron is believed to be related to the stabilization of the more ductile f.c.c. form. Thus the alloy of the present invention stabilizes the Laves phase and reduces the tendency of the h.c.p. cobalt formation by an increase in the iron level and stabilization of Laves phase and the h.c.p. form of cobalt through the reduction of Si to as low a level as is commercially possible.

Thus the above data, typical of that obtained on the heats of the table show unexpectedly that small variations in the elements Si and Fe, at carbon levels of less than about 0.1 weight percent can result in significant changes in tensile and bend ductility for cobalt base Co-Cr-W alloys during prolonged exposure in the range of 1200-2000" F. The improvement in room temperature ductility of the aged alloys to which this invention relates through control of the precipitating phases is an unexpected and significant contribution to the art and understanding of problems relating to cobalt base alloys. The data has shown that the effect of maintaining silicon at less than 0.5 weight percent is significantly stronger than the effect of iron. It is contemplated that maintaining silicon within the specified range along with grain size adjustment, could allow the reduction of the iron content to a level less than the specifically preferred range 2-5 weight percent.

Although the present invention has been described in connection with the specific examples, it will be recognized by those skilled in the art the variations of which the present invention is capable without varying from its scope.

What is claimed is:

1. An improved cobalt base alloy consisting essentially of, by weight, the elements Cr, W and Ni in a total amount of 42-48%; elements selected from the group consisting of Si and Zr being present only in amounts such that the Si when included is no more than 0.5% and the Zr when included is of no more than 0.2%; 0.05-0.15% C; 2-5% Fe; about 1-2% Mn; with the balance cobalt.

2. An improved cobalt base alloy consisting essentially by weight, of 19-21% Cr; l4-l6% W; 9-11% Ni; 1-2% Mn; 0.05-0.15% C; 2-5% Fe; Si in an amount no more than 0.5% with the balance cobalt.

3. An improved cobalt base alloy consisting essentially 'by weight, of 19-21% Cr; 14-16% W; 9-1l% Ni; 1-2% Mn; 0.05-0.1% C; 3-5% Fe; Si in an amount no more than 0.5% with the balance cobalt.

4. An improved cobalt base alloy consisting essentially of, by weight, l9-21% Cr; 1416% W; 9-11% Ni; 1-2% Mn; 0.05-0.15% C; 2-5% Fe; elements selected from the group consisting of Si and Zr being present only in amounts such that the Si when selected is no more than 0.5% and the Zr when selected is no more than 0.2% with the balance cobalt.

References Cited UNITED STATES PATENTS 2,684,299 7/1954 Binder 75-171 2,704,250 3/1955 Payson 75-171 2,996,379 8/1961 Faulkner 75-171 RICHARD O. DEAN, Primary Examiner. 

1. AN IMPROVED COBALT BASE ALLOY CONSISTING ESSENTIALLY OF, BY WEIGHT, THE ELEMENTS CR, W AND NI IN A TOTAL AMOUNT OF 42-48%; ELEMENTS SELECTED FROM THE GROUP CONSISTING OF SI AND ZR BEING PRESENT ONLY IN AMOUNTS SUCH THAT THE SI WHEN INCLUDED IS NO MORE THAN 0.5% AND THE 